Magnetic discs are used in disc drive storage systems for storing digital data and in spin-stands as components on which transducing heads are tested. Disc drives are commonly used in work stations, personal computers, laptops and other computer systems to store large amounts of data on one or more magnetic discs in a form that can be made readily available to a user. The discs are mounted to a spindle motor which rotates the discs at a high speed about an axis. An actuator assembly supports and positions an array of transducing heads over surfaces of the discs to write information to or read information from the discs.
Spin-stands are devices that are used to test transducing heads before they are placed in a disc drive. As in a disc drive, a spin-stand includes a disc that is mounted to a spindle motor which rotates the disc at a high rate of speed about an axis. Also, the spin-stand includes an actuator assembly that supports the head over the disc surface and moves the head to a desired location. The spin-stand allows a series of tests to be performed on the transducing head including, for example, error-rate testing, pulse width-fifty testing, track average amplitude testing, and track scan testing.
The discs used in disc drives and spin-stands generally include circular data tracks which extend circumferentially around each disc. These data tracks are defined by radially extending servo tracks that contain servo information. The servo information defines the boundaries and centerlines of each of the tracks.
Disc drives, and more recently spin-stands, utilize servo systems to control the position of a head relative to the data tracks using the servo information. As a head moves over a surface of a disc, the head reads the servo information and produces an output signal that indicates its position relative to the servo tracks. The output signal is demodulated and compared with a reference position signal relating to a desired head position to produce a position error signal (PES). The PES is provided to a servo controller that produces a control signal which is used to control an actuator mechanisms of the disc drive or spin-stand to move the head toward the desired position or data track. Once the head is positioned over the desired data track, the servo system allows the head to follow the track using the servo information.
The servo tracks are typically written after the discs have been installed in a disc drive or spin-stand with a servo track writer. These “post-written” tracks are substantially concentric with the axis of rotation of the disc on which they are written, since the axis of rotation remains constant from when the servo information is written to when the servo information is used to perform track following. However, uncontrolled factors such as bearing tolerances, spindle resonances, and the like, tend to introduce errors in the location of the servo information. As a result, each track is typically not perfectly concentric with the axis of rotation of the disc, but rather exhibits certain random, repeatable variations which are sometimes referred to as repeatable runout (RRO). This slight misalignment is exhibited in a periodic PES.
There is a continuing trend in the disc drive industry to provide successive generations of disc drive products with ever increasing data storage capacities and data transfer rates. Because the amount of disc surface available for the recording of data remains substantially constant (or even decreases as disc drive form factors become smaller), substantial advancements in areal recording densities, both in terms of the number of bits that can be recorded on each track as well as the number of tracks on each disc (measured as tracks per inch or TPI), are continually being made in order to facilitate such increases in data capacity. One way to improve storage capacities is to improve the writing of the servo patterns on the discs. To this end, servo information is written on the discs prior to their installation in a disc drive or on a spin-stand using highly precise servo writers.
Although these “pre-written” tracks can result in an increase in the TPI of the disc, large RRO will result due to large eccentricity between the data tracks and the axis of rotation of the disc. This eccentricity primarily stems from the re-mounting of the disc to the spindle motor of the disc drive or spin-stand. In addition, the RRO that affects disc drives and spin-stands using post-written discs will also be present when these pre-written discs are used. However, the RRO correction methods mentioned above are not suitable for compensating a PES corresponding to the large RRO resulting from the use of discs having pre-written servo tracks. This is due, in part, to the large sinusoidal current that would have to be injected into the PES to compensate the large adjustments to the position of the head that would be necessary to follow a particular data track as defined by the pre-written servo tracks.
The present invention addresses these and other problems, and offers other advantages over the prior art.